1. Field of the Invention
This invention relates to a projection cathode ray tube in which an image on a phosphor layer is enlarged and projected on a screen, located at a given distance ahead through a projection lens unit, in front of the phosphor layer.
2. Description of the Related Art
The U.S. Pat. No. 4,642,695 discloses an example for the improvement in efficiency for gathering luminous flux into the projection lens unit from each of the monochromatic cathode ray tubes.
In a conventional cathode ray tube, light emitted from a phosphor screen is in a state of highly close to, what is called, completely scattered light. In a projection television apparatus, a luminous flux emitted from the phosphor screen within the divergent angle of 30.degree. is exclusively utilized, while the other becomes useless luminous flux. According to the above-disclosed example, the brightness of an image on a screen of the projection television is greatly enhanced by converging 30% or more of all luminous flux emitted from elements of the light-emitting phosphors into a cone-shaped area having the divergent angles within -30.degree. to 30.degree..
Further, according to the publication of the Japanese Laid-open Patent Application No. 61-273837, there is disclosed a projection type cathode ray tube having a multilayered interference filter composed of alternately superimposed high refractive index material layers and low refractive index material layers, disposed between the inner surface of a face plate pane of the cathode ray tube and a luminescent material layer provided at the innermost of the cathode ray tube, for example. As examples of structural components, tantalum pentoxide (Ta.sub.2 O.sub.5) or titanium dioxide (TiO.sub.2) is used as the high refractive index material, whereas silicon dioxide (SiO.sub.2) or magnesium fluoride (MgF.sub.2) is used as the low refractive index material. In general, the multilayered interference film is formed of the low refractive index material layers and the high refractive index material layers alternately superimposed in six layers or more.
In a projection cathode ray tube having a conventional multilayered interference film disposed upon the inner surface of the face plate pane, the optical thickness nd of each layer of the multilayered interference film is between 0.2 and 0.3 .lambda..sub.f in accordance with the example disclosed in the publication of the Japanese Laid-open Patent Application No. 61-273837. Specifically, according to this example, the projection cathode ray tube comprises a multilayered interference film composed of six or more layers, the optical thickness nd of which is within 0.23 to 0.25 .lambda..sub.f, namely, around .lambda..sub.f /4. Here, the following equation is obtained: EQU .lambda..sub.f =p.times..lambda.
where .lambda. denotes a desired central wavelength selected from the emission spectrum of the luminescent material; p, a value in the range from 1.18 to 1.32. As another example, the publication of the Japanese Laid-open Patent Application No. 61-39349 discloses a projection cathode ray tube having a multilayered interference film composed of layers superimposed within 14 to 30 layers, the optical thickness of which is almost the same as that of the aforementioned example.
The cathode ray tube is generally fabricated through the manufacturing processes as shown in FIG. 2. In these processes, cathode ray tubes are typically manufactured passing through the heat processing steps three times, the temperature of which is approximately within the range of 350.degree. C. to 450.degree. C., such as baking, frit sealing and exhaust steps. In view of the heat processing, before passing through the above-described manufacturing steps, the cathode ray tubes have previously been processed by baking at the temperature ranging from 450.degree. C. to 500.degree. C. so as to secure the multilayered interference film which has been deposited by the evaporation.
However, in the existing multilayered interference film mentioned above, the number of layers comprised therein becomes a great number; for example, it amounts to six or more layers, sometimes up to 14 to 30 layers. Therefore, there is a disadvantage that because of the difference in the thermal expansion coefficients between the high refractive index material and the low refractive index material of the multilayered interference film, a thermal distortion occurs at each layer of the multilayered interference film due to the aforementioned heat processing, thereby causing cracks or a film separation.